Ion implanters are commonly used in the production of semiconductor wafers and other devices, such as solar cells. An ion source is used to create an ion beam, which is then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits.
A block diagram of a representative ion implanter 100 is shown in FIG. 1. One skilled in the art will recognize that the ion implanter 100 is only one of many ion implanter designs. An ion source 110 generates ions of a desired species. In some embodiments, these species are atomic ions, which may be best suited for high implant energies. In other embodiments, these species are molecular ions, which may be better suited for low implant energies. These ions are formed into a beam, which then passes through a source filter 120. The source filter is preferably located near the ion source. The ions within the beam are accelerated/decelerated in column 130 to the desired energy level. A mass analyzer magnet 140, having an aperture 145, is used to remove unwanted components from the ion beam, resulting in an ion beam 150 having the desired energy and mass characteristics passing through resolving aperture 145.
In certain embodiments, the ion beam 150 is a spot beam. In this scenario, the ion beam passes through a scanner 160, which can be either an electrostatic or magnetic scanner, which deflects the ion beam 150 to produce a scanned beam 155-157. In certain embodiments, the scanner 160 comprises separated scan plates in communication with a scan generator. The scan generator creates a scan voltage waveform, such as a sine, sawtooth or triangle waveform having amplitude and frequency components, which is applied to the scan plates. In a preferred embodiment, the scanning waveform is typically very close to being a triangle wave (constant slope), so as to leave the scanned beam at every position for nearly the same amount of time. Deviations from the triangle are used to make the beam uniform. The resultant electric field causes the ion beam to diverge as shown in FIG. 1.
In an alternate embodiment, the ion beam 150 is a ribbon beam. In such an embodiment, there is no need for a scanner, so the ribbon beam is already properly shaped.
An angle corrector 170 is adapted to deflect the divergent ion beamlets 155-157 into a set of beamlets having substantially parallel trajectories. Preferably, the angle corrector 170 comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function.
Following the angle corrector 170, the scanned beam is targeted toward the workpiece 175. The workpiece is attached to a workpiece support. The workpiece support provides a variety of degrees of movement.
The workpiece support is used to both hold the wafer in position, and to orient the wafer so as to be properly implanted by the ion beam. Another critical function of the workpiece support is to provide cooling to the workpiece 175, as the workpiece may become hot during the implantation process. The workpiece support may be cylindrical in shape, such that its top surface is circular, so as to hold a disc-shaped wafer. Of course, other shapes are possible. To effectively hold the wafer in place, most workpiece supports typically use electrostatic force. By creating a strong electrostatic force on the upper side of the support, also known as the electrostatic chuck, the wafer can be held in place without any mechanical fastening devices. This minimizes contamination and also improves cycle time, since the wafer does not need to be unfastened after it has been implanted. These chucks typically use one of two types of force to hold the wafer in place: coulombic or Johnson-Rahbeck force.
As seen in FIG. 2, this chuck 200 traditionally consists of several layers. The first, or top, layer 210, which contacts the wafer, is made of an electrically insulating or semiconducting material, such as alumina, since it must produce the electrostatic field without creating a short circuit. In some embodiments, this layer is about 4 mils thick. For those embodiments using coulombic force, the resistivity of the top layer, which is typically formed using crystalline and amorphous dielectric materials, is typically greater than 1014 Ω-cm. For those embodiments utilizing Johnsen-Rahbeck force, the volume resistivity of the top layer, which is formed from a semiconducting material, is typically in the range of 1010 to 10 Ω-cm. The term “non-conductive” will be used to describe materials in either of these ranges, and suitable for creating either type of force. The coulombic force can be generated by an alternating voltage (AC) or by a constant voltage (DC) supply.
Directly below this layer is a layer of electrically conductive material 220, which contains the electrodes that create the electrostatic field. This layer is made using electrically conductive materials, such as silver. Patterns are created in this layer, much like are done in a printed circuit board to create the desired electrode shapes and sizes. FIG. 3 is a front plan view of a first embodiment of an electrode pattern for a platen. FIG. 4 is a front plan view of a second embodiment of an electrode pattern of a platen. Below this conductive layer 220 is a second insulating layer 230, which is used to separate the conductive layer 220 of the platen from the lower portion 240.
The lower portion 240 is preferably made from metal or metal alloy with high thermal conductivity to maintain the overall temperature of the chuck within an acceptable range. In many applications, aluminum is used for this bottom layer. In some embodiments, this bottom layer has two separate aluminum portions. The lower portion is thick and contains fluid passageways. Typically, the top surface of an aluminum block is machined to introduce channels 250 through which coolant is passed. The coolant can be any suitable fluid, including water and de-ionized water. A much thinner second aluminum plate is formed to act as a lid for this thicker aluminum block, providing a cover for these machined passageways. These two aluminum portions are bonded together to form the thermally conductive lower layer of the electrostatic chuck. This layer and the previously described electrically non-conductive layer are then mechanically affixed together, such as by epoxy, brazing material or other adhesive technique.
In addition to the fluid conduits that are used to cool the platen, there may be other conduits 260 used to carry gas to the top surface of the platen 200. These conduits are used to pump gas, known as backside gas, to the underside of the workpiece. This gas helps improve the thermal transfer between the workpiece and the platen.
FIG. 3 shows a top plan view of one embodiment of a platen 300. In this figure, six distinct electrodes 310, each in a spiraling shape, are visible, each separated from the others by a thin insulating boundary 315. The electrodes may be driven by a power supply, generating a three phase signal, such that there is a A+/A− pair, a B+/B− pair, and a C+/C− pair. During the first phase, the A+/A− pair is energized. During the second phase, the B+/B− pair is energized, and during the third phase, the C+/C− pair is energized. This continuous cycling of voltage around the platen 200 creates the electrostatic force needed to hold the workpiece clamped to the platen.
Located within the electrode pattern are a number of gas holes 320, which supply back side gas to the volume between the platen 300 and the workpiece. The gas cooling hole distribution is determined based on desired gas flow and pressure uniformity. Greater gas flow and pressure improves the uniformity of thermal transfer from the wafer to the platen which in turn improves temperature uniformity of the wafer. However, because the gas pressure tends to force the workpiece away from the platen 300, these gas holes 320 are typically located a distance away from the outer circumference of the platen 300, to insure sufficient electrode area exists to hold the workpiece to the platen. The pressure from the injected gas tends to force the workpiece away from the platen. By extending the electrodes beyond the gas holes, the outer edge of the workpiece is held down, thereby stopping the gas from escaping via the outer edge of the workpiece. Thus, the electrostatic forces created by the three electrode pairs counteracts the forces created by the introduction of back side gas.
FIG. 4 shows a top plan view of a second embodiment of an electrode pattern on a platen 400. As in FIG. 3, there are six distinct electrodes 410, arranged in three pairs, as described above. A number of gas holes 420 are arranged around the platen. A portion of the electrode pattern extends beyond the outer ring of gas holes to secure the outer edge of the workpiece to the platen.
While this configuration has been used for some time, there are a number of drawbacks associated with it. First of all, since there is only a single electrode layer in the platen, each of the six electrodes must be contiguous. In other words, the portion of the platen that is energized during any particular phase must be connected. This requirement leads to irregularly shaped electrode patterns, as can be seen in both FIG. 3 and FIG. 4. The reason for this is an attempt to balance this requirement with the need to spread the electrodes evenly across the platen to maximize the holding effect. Secondly, the gas flow to the underside of the workpiece may be less than optimal, due to the location constraints described above. It would therefore be beneficial if the electrodes could be separated into various non-connected portions to optimize their holding ability. Finally, it would be beneficial if the gas flow to the underside of the workpiece were improved.